Wafer base for silicon carbide semiconductor devices, incorporating alloy substrates

ABSTRACT

A semiconductor wafer base is disclosed which is suitable for fabrication of devices in silicon carbide, comprising a single crystal substrate which is a transition metal carbide alloy having cubic crystal structure and an unpolytyped, single crystal 3C-silicon carbide overlay epitaxially related to the substrate. Preferably, the substrate is an alloy of two or more of titanium carbide, tantalum carbide, vanadium carbide, and niobium carbide, with alttice parameter differing from 3C-silicon carbide by less than about 1%. Use of the transition metal carbide alloys enables the preparation of large, single crystal substrates free from cracks, dislocations, or other defects, suitable for epitaxial deposition of 3C-silicon carbide. The 3C-silicon carbide epitaxial overlay may be deposited by any suitable technique, including chemical vapor deposition and reactive evaporation, and may be doped with n- or p-type dopants. The 3C-silicon carbide is useful for fabricating semiconductor devices for use at high temperatures, high powers, and in corrosive environments.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor devices and methods offorming same and more particularly to a semiconductor device wafer basewherein a silicon carbide overlay is deposited on a suitable substrate.

2. Description of the Related Art

An extensive technology of semiconductor devices has been developedbased upon the properties of single crystal silicon and other similarmaterials which may be doped, heat treated, and otherwise processed toproduce adjacent layers and regions of varying electroniccharacteristics. The use of devices produced by silicon technology isgenerally limited to operation at ambient or, at most, moderatelyelevated temperatures and in non-corrosive, inert atmospheres. Thetemperature limitation is a consequence of the intrinsic behavior ofsilicon at high temperatures and of the rapid diffusion of dopants orimpurity species, which in turn can substantially alter the character ofthe fabricated semiconductor device. The limitation to relatively inertenvironments results from the high chemical reactivity of silicon inmany corrosive environments, which also can alter the character of thefabricated device. Silicon devices are also limited as to power level,frequency, and radiation tolerance by the materials used therein.

For some applications, the temperature, environmental, and other uselimitations on silicon devices may be overcome by the use of propercooling and packaging techniques. In other applications, theselimitations have prevented the use of silicon for integrated circuittechnology. For example, in many spacecraft and aircraft applications,elevated temperatures are encountered, and it is not always possible toinsure that adequate cooling will be provided. In high powerapplications, internal thermal transients in devices otherwise operatingat ambient temperature can rapidly destroy the operability of the deviceunless extensive cooling is provided. Such cooling requires that thedevice be larger in size that might otherwise be necessary, in partdefeating the purpose of the integrated circuit technology.

There has therefore been an ongoing, but as yet not fully successful,search over a period of twenty years to identify and develop asemiconductor technology based in other materials. Such a technologywould desirably allow the fabrication of devices for use at highertemperatures such as, for example, the range of at least about 400° C.to 600° C., and in applications not amenable to the use of silicon.Because corrosive effects can be greatly accelerated at elevatedtemperatures and pressures, any such materials and devices must alsoexhibit excellent corrosion resistance at the elevated use temperaturesand over a range of pressures from vacuum to many atmospheres. Somegenerally desirable characteristics of such materials have beenidentified, including large band gap, good electrical conductivity, highbreakdown electric field, low dielectric constant, ability to be dopedto produce regions of varying electronic characteristics, a high meltingtemperature, good strength at operating temperatures, resistance todiffusion by undesired foreign atoms, good thermal conductivity, thermalstability, chemical inertness, and the ability to form ohmic externalcontacts.

Silicon carbide is a candidate material that can meet the requirementsenumerated above. SiC is the only compound species that exists in theSi-C system, but it can occur in many crystalline polytypes. The lonecubic polytype is referred to as 3C-silicon carbide, or also as beta-SiCor β-SiC. 3C-SiC crystallizes in the zinc-blende structure. About 170hexagonal and rhombohedral polytypes are known, which are referred tocollectively as "alpha-SiC." The most common is 6H, which has ahexagonal crystal structure.

3C-SiC has many properties superior to silicon and to gallium arsenide.Among the desirable properties of 3C-SiC are high breakdown voltage, arelatively large bandgap (2.3 eV at ambient temperature) and thermalconductivity over three times higher than that of silicon at ambienttemperature. 3C-SiC is resistant to the diffusion of impurity speciesand may be processed by several techniques similar to those used insilicon device technology. The high thermal conductivity and highbreakdown field obtainable for 3C-SiC predict that integration of highdensity devices can be achieved in this material.

Two figures of merit that are particularly relevant to the potentialapplications of 3C-SiC have been devised to compare semiconductormaterials. Keyes Figure of Merit compares semiconductor materials on thebasis of switching speeds that could in principle be obtained withtransistors fabricated in the material. In this case a high limitingvelocity allows high speed devices. Small, closely spaced devices have ahigh density of power dissipation (heat production). The heat flows fromthe device by conduction through the semiconductor material therebyencountering a thermal resistance that is inversely proportional to thesize of the device. A lower limit is set on the device size by themaximum permissible thermal resistance and, thus, a high thermalconductivity σ_(T) is greatly desirable in the device material. Keyesfigure of merit is σ_(T) (V_(sat) /K)^(1/2) where K is the dielectricconstant of the material. Johnson's Figure of Merit comparessemiconductor materials on basis of high frequency and high powercapabilities for discrete devices. The basic limitation on varioustransistor characteristics is set by the product of the breakdownelectric field E_(B) and the saturated electron velocity V_(sat), i.e.the velocity at which an electron has sufficient energy to emit anoptical photon. Johnson's figure of merit is (E_(B) V_(sat) /π)². Acomparison of Johnson's and Key's figures of merit for varioussemiconductor materials vs. silicon (from "Critical Evaluation of theStatus of the Areas for Future Research Regarding the Wide Band GapSemiconductors Diamond, Gallium Nitride and Silicon Carbide," Davis etal., Materials Science and Engineering, B1 (1988), pp. 77-104) shows thepotential usefulness of 3C-SiC for high temperature, high powersemiconductor devices:

    ______________________________________                                                Johnson's Figure of Merit                                                                      Keyes' Figure of Merit                               Material                                                                              (ratio to Si)    (ratio to Si)                                        ______________________________________                                        Silicon 1.0              1.0                                                  GaAs    6.9              0.456                                                InP     16.0             0.608                                                GaN     281.6            1.76                                                 6H-SiC  695.4            5.12                                                 3C-SiC  1137.8           5.8                                                  ______________________________________                                    

Mehdi, Haddad and Mains have performed an numerical simulation analysisthat shows that 3C-SiC may be especially useful for high frequency, highpower devices such as impact avalanche transit-time (IMPATT) diodes (J.Appl. Phys. 64, 1533(1988)). 3C-SiC devices theoretically can producesignificantly higher powers than Si and GaAs devices at comparablefrequencies.

An additional advantage of 3C-SiC is that many device processing stepsthat have been developed for Si may be used with very littlemodification. Among the 3C-SiC similarities to Si that are relevant todevice processing are: 3C-SiC has a shallow donor dopant (N) which canbe incorporated controllably during epitaxial growth or by ionimplantation; 3C-SiC has a shallow acceptor dopant (Al) which can beincorporated controllably during epitaxial growth or by ionimplantation; it is possible to grow thermal SiO₂ with good electricalproperties (prototype 3C-SiC MOSFETs operating at 650° C. have beenfabricated; see Davis et al. referenced above); 3C-SiC is not etched byHF; 3C-SiC can withstand temperatures up to 1550° C. during homo- orheteroepitaxial growth. Silicon carbide may thus be processed by severaltechniques similar to those used in silicon device technology, and inmany instances silicon carbide devices may be substituted at moderateand low temperatures for silicon devices. Silicon carbide semiconductordevice technology therefore offers the opportunity for supplementing,and in some instances replacing, conventional silicon device technology.

The chief obstacle to the production of 3C-SiC devices has been thedifficulty of producing unpolytyped SiC single crystals of sufficientsize to allow fabrication of devices. SiC does not melt congruently;3C-SiC converts to an alpha polytype above 1600° C. and dissociates at2830° C. under 35 atmospheres of argon.

Small bulk crystals of 3C-SiC have been made, ˜2 mm×2 mm. These smallcrystals showed that devices would indeed be extremely useful but notbig enough realistically to fabricate device arrays as is done withsilicon. Therefore although device potential has been demonstrated on avery small scale, there have been no demonstrations of devices on alarge enough scale to be useful. Small scale devices fabricated fromvapor-grown SiC crystals included uv flame detectors, grown junctiondiode rectifiers, and thermistors.

An alternative to growing bulk crystals and fabricating devices byhomoepitaxy is preparation of wafer bases by heteroepitaxiallydepositing an overlay onto a foreign substrate. A high quality, singlecrystal substrate with a good lattice match is required for the successof this approach. Previous efforts tried this approach using 3C-SiCsingle crystal substrates, but large 3C-SiC single crystals have notbeen prepared without included alpha polytypes. The 3C-SiC overlaycarried the defects forward. Silicon substrates have been tried, but thecoefficient of thermal expansion of silicon is about 8% different andlattice parameters differ by 20%. Lattice mismatch results in formationof numerous microtwins, intrinsic stacking faults and antiphaseboundaries (APBs). Use of the <100> silicon surface, slightly off-axis,reduces the number of APBs but does not completely eliminate them.Unfortunately other types of defects are not eliminated by thisprocedure. Carburization of the silicon surface has been tried in anonly partially successful attempt to ameliorate these problems. 6H-SiC,with its hexagonal crystal structure, also has proven unsuitable as asubstrate, because APBs are formed when it is attempted to formepitaxial overlays of 3C-SiC (for example, see H. S. Kong, J. W.Palmour, J. T. Glass and R. F. Davis, Appl. Phys. Lett., 51,442 (1987)).Stacking faults typically emanate from these APBs. APBs and theresulting stacking faults degrade semiconductor performance, causinghigh parasitic resistances that would prevent high frequency operationof semiconductor devices. Alpha SiC has many polytypes, andinhomogeneous polytypes will lead to inhomogeneous heteroepitaxial 3C-SiC films. Various metals that have cubic crystal structures have beentried, such as molybdenum with and without a liquid metal intermediatelayer. However, pure metals have problems with diffusion of the Si and Cinto the metal. The cubic metals that are potentially useful on thebasis of their lattice dimensions are carbide-formers. Carbon diffusesinto the metal surface and forms metal carbides; it is difficult tocontrol Si:C stoichiometry during growth. Most high-melting metals havethe body-centered cubic (bcc) structure; bcc metal substrates would leadto many APBs in the 3C-SiC overlay. Also, metal silicide formation canoccur at the interface, preventing true epitaxy.

Therefore, to summarize the many problems with substrates:

no large single crystals

large lattice parameter mismatch

thermal expansion mismatch

defects in the substrate cause unsatisfactory surface morphology andcrystal quality of resulting 3C-SiC overlay

Larger scale devices require large area 3C-SiC epitaxial layers suitablefor device or IC processing. These epitaxial layers must be able to beformed reproducibly, in single crystals having lateral dimensionsgreater than several square millimeters by a fabrication technique thatdoes not adversely influence the key properties of the 3C-SiC overlayer,that is, its thermal stability, corrosion resistance, and desirableelectrical properties.

Bunshah, Parsons and Stafsudd attempted to solve the substrate problemby using cubic TiC (or, alternatively, ZrC, WC, TaC or ScN) singlecrystals as substrates, most especially favoring TiC because: (1) TiChas a good lattice match to 3C-SiC (TiC has a lattice parameter of 4.33Å compared to 4.36 Å for 3C-SiC, the lattice parameters thus differingby only 0.7% at ambient temperatures; (2) TiC has only one polymorph andno polytypes; (3) TiC and 3C-SiC have an acceptable coefficient ofthermal expansion match, with TiC having a slightly larger thermalexpansion. This larger thermal expansion is helpful, since it places the3C-SiC film into slight compression upon cooldown after film growth,rendering formation of microcracks and pinholes less likely. In general,the surface morphology of thick (>1 μm) epilayers tends to be rough whenthe epilayers are tensionally loaded. If the TiC is of good quality, theresulting 3C-SiC crystals have a very low density of stacking faults andother crystalline defects.

Additionally, Ti is electrically inert in SiC. At the high temperaturesof 3C-SiC growth (1200°-1600° C.), some interdiffusion can be expectedto occur, but without deleterious electrical effects. If desired,interdiffusion of Ti into the SiC can be prevented by saturating thesurface of the TiC with carbon immediately before SiC growth.

Any TiC crystallographic orientation will support heteroepitaxy of3C-SiC. Growth on the <111> surface is preferred and off-axis slicingcan improve lattice matching. TiC is a good electrical conductor with awork function of ˜3eV, allowing for the possibility of fabricating noveldevices such as permeable base transistors. TiC has higher thermalconductivity than SiC or Si, and thus thermal gradients are minimizedand thermal energy dissipation is enhanced.

Despite these potentially significant advantages, serious problems areencountered in preparing single crystal TiC of adequate size andquality. Large area single crystals of TiC are unavailable, and thislack is hindering 3C-SiC device development.

Carbide single crystals are grown by float zone techniques because thematerials melt at such high temperatures that it has not been possibleto realize uncontaminated crystal growth by methods that require acontainer. Alternative methods that could potentially be used for TiCsingle crystal growth include other "crucible-less" techniques such asskull melting. The same problems and advantages as are confronted infloat-zoning would apply to these methods.

The float zone process involves formation of a molten zone andsubsequent movement of the zone along a feed rod which is made frompressed and sintered powders of the substance that one is trying torecrystallize. Float zoning has the advantage that no contamination froma crucible occurs. Large crystals may usually be prepared by thistechnique.

TiC single crystals are grown by float zone techniques because of thehigh melting point of TiC. Problems that hinder TiC crystal growthinclude the following:

(1) cracks are prone to form,

(2) subgrain boundaries are usually observed and these generate grainboundaries in the epitaxial 3C-SiC overlayer,

(3) pinholes form, creating pinholes in the SiC overlayer, and

(4) stacking faults and other dislocations are difficult to avoid infloat zoned TiC. These defects create poor morphology and crystalimperfections in the epitaxial overlayer of 3C-SiC.

Several possible explanations for the problems encountered in growingTiC single crystals have been advanced. The molten zone is at a veryhigh temperature because of the high melting point (3160° C.) of TiC;therefore enormous thermal stresses are present during growth. Thestresses in the "cooldown" region near the molten zone are severe, fromthe molten temperature of >3000° C. down to about 1500° C. Below1500°-800° C., cracking occurs. It is generally believed that below 800°C., dislocations do not move through the crystal. These thermal stressescause dislocations and defects. It is difficult to heat a large feed roduniformly, exacerbating the thermal stress problem that already existswith cooldown. It will be very hard indeed to heat a 2 inch diameter roduniformly, and this is the size that must be targeted for economicaldevice fabrication processes. Arcing is a problem in float zoning athigh powers. In TiC float zone recrystallization, a very large amount ofrf power is required; the growing crystal is blanketed with helium toprevent arcing, but the helium has a high thermal conductivity andcarries enough heat away from the rod to significantly contribute to thethermal stress problem. TiC has a high vapor pressure at its meltingpoint and can vaporize and coat the induction coils, increasing thelikelihood of arcing.

Solid solutions of cubic carbides are known. A prominent characteristicof the cubic monocarbides is their extensive mutual solubility. Ingeneral, the cubic carbides are mutually soluble in all concentrationranges when the covalent radii of the metal atoms differ by less than10%.

Hollox et al. (U.S. Pat. No. 3,661,599), in efforts to make hightemperature structural materials, alloyed vanadium carbide into TiC.Feed rods were prepared by mixing TiC and VC powders in specified ratiosand pressing and sintering the rods. A molten zone was passed throughthese prepared feed rods (a process referred to as "zone refining").Previously, cubic carbide structural materials in powder mixtures hadbeen sintered and did not form fully dense solids. These less than fullydense carbide solids thus were not optimally strong for the structuralapplications that Hollox et al. were targeting. U.S. Pat. No. 3,661,599teaches alloying powders followed by zone refining to lead to fullydense, single phase materials, which had several advantages. The zonerefined alloys were extremely strong, judged by compressive yieldstrength (CYS). Most significantly, the alloys were stronger than thepure materials. The alloy containing 25% TiC: 75% VC by weight was thestrongest at high temperatures (>1200° C.), with the 50:50 alloy closebehind. Above 1400° C., 75% TiC: 25% VC has CYS 1/3 of 25% TiC: 75% VC.The TiC and VC pure carbides have much lower CYS, which fell offmarkedly at temperatures above about 1200°-1400° C.

There are several possible rationalizations for the observation that themixture may be much stronger than the single pure component.Dislocations travel through a mixed lattice less easily. Higher yieldstrength may imply less defect formation under the highest thermalstresses. It is worth noting that the CYS improvement was observed inthe temperature regime where the thermal stresses are the greatestduring float zone cool-down. The allows were easier to float zone thanthe pure carbides. Alloying decreased the number and size of cracks inthe resulting crystalline boule. The carbide alloys formed by theprocess of U.S. Pat. No. 3,661,599 could be polycrystalline or singlecrystal, depending on the rate the molten zone was passed along the rod.

It is known that the dislocation density of semiconductor compounds ofcolumns III-V and II-VI of the periodic chart can be reduced by properintroduction of alloying atoms into the compounds. In Sher U.S. Pat. No.4,607,202 and Mooney et al. U.S. Pat. No. 4,568,795, it is taught thatthe dislocation density of III-V and II-VI semiconductor compounds canbe reduced by alloying such semiconductors with isoelectronic impurityatoms forming bond lengths with the semiconductor atoms that are lessthan the bond lengths between atoms of the semiconductor. The alloyingatoms can be added in relatively low amounts only, so that thesolubility limit of the host semiconductor compound cannot be exceeded.This value is insufficient to eliminate dislocations totally.

Parsons and Stafsudd (International Patent Publication WO 89/06438) havedescribed alloying of silicon carbide with one or more of the carbidesof Ti, Hf, Zr, V, Ta, Mo, W, and Nb, with Ti, Hf, and Zr preferred. Byselecting appropriate proportions of metal carbide and SiC, the alloy'sbandgap may be tailored to any desired level between the bandgaps of themetal carbide and SiC. Semiconductor devices are formed by epitaxiallygrowing a layer of the new alloy upon a latticematching substrate,preferably TiC. While retaining the benefits of single-bandgap 3C-SiC,the new alloys may enable various electrical devices that cannot beachieved with 3C-SiC, and also have a potential for bandfoldedsuperlattices for infrared detectors and lasers. Solid solutions of3C-SiC with aluminum nitride to modify the bandgap have been reported("Synthesis and study of epitaxial layers of (SiC)_(1-x) (AIN)_(x)wide-gap solid solutions, "Nurmagomedov, et al., Sov. Tech. Phys. Lett.12(9), p. 431 (1987)) and show promise as materials for optoelectronicsand acoustoelectronics. These solid solutions are thought to retaincubic crystal structure at very low AIN concentrations. The problems ofsubstrate suitability and producibility that plague 3C-SiC devicedevelopment will also hinder the development of devices based on thesilicon carbide alloys and solid solutions. Hence, while the discussionherein refers to 3C-SiC, it will be appreciated that the sameconsiderations and advantages of the present invention will apply to theSiC alloys and solid solutions as well.

It is an object of the present invention to provide a device wafer baseof 3C-SiC overlaid upon a substrate which is a single crystal havingdimensions greater than several mm so that arrays of devices can beprocessed and prepared thereon. Another object of the invention is toprovide a method for fabricating the wafer base using alloys suitable toform the single crystal substrate. Another object of the invention is toprovide a method for making single crystals suitable for serving as thesubstrates for 3C-SiC overlay to form a device wafer base. Other objectsand advantages of the present invention will be more apparent from theensuing disclosure and the appended claims.

SUMMARY OF THE INVENTION

The present invention relates in one aspect to electronic device waferbases comprising epitaxial thin films of 3C-SiC on suitable substrates,useful for fabrication of semiconductor devices.

The substrates useful for 3C-SiC epitaxial overlay must have certainproperties. The substrate must be readily fabricated as relatively largesingle crystals, preferably as boules with dimensions of at least 0.5inches and preferably approximately 2 inches in diameter or larger. Thesubstrate must have a surface suitable for epitaxial deposition of3C-SiC. The surface must not adversely influence the desirableproperties of the SiC overlay by manifesting any defects orinhomogeneities that would be carried forward into the 3C-SiC epitaxiallayer. The crystal structure of the substrate must be compatible withthe cubic crystal structure of 3C-SiC. In this regard, substrates havingthe face-centered cubic (fcc) crystal structure are particularly useful.The crystalline lattice of the substrate must have a good lattice matchto 3C-SiC. Thermal stability of the substrate at the 3C-SiC growthtemperatures is required. The thermal expansion of the substrate must berelatively close to that of 3C-SiC. The substrate may be prepared byknown techniques such as float zone crystallization. The 3C-SiCepitaxial overlay upon the crystal substrate to form the device waferbase may be fabricated by known techniques such as chemical vapordeposition, reactive evaporation, or liquid phase epitaxy.

The substrate upon which the epitaxial layer of 3C-SiC is depositedcomprises an element or compound capable of single crystal growth inadequate size for useful device fabrication processes, at least 0.5inches and preferably about 2 inches or larger in diameter, as crystalrods for lateral slicing. The substrate is preferably a metal carbidealloy having a cubic crystal structure. Alloys of transition metalcarbides, with any non-cubic component present in concentrationssufficiently low that the alloy retains a cubic crystal structure, areparticularly well-suited for the present invention. Metal carbidesuseful in the present invention are WC, VC, MoC, TiC, TaC, HfC, NbC, andZrC, with alloys containing two or more of VC, TiC, TaC, and NbC beingespecially preferred. VC/TiC alloys with VC mole percent ranging from5-90% and NbC/VC alloys, with the alloy having a 2:1 mole ratio ofNbC:VC, are especially preferred because of their good lattice parametermatch to 3C-SiC and increased ease of float zone recrystallization.Alloys which include WC or MoC, which form a hexagonal crystal structurein their pure state, may be used when the concentration of WC or MoC islower than about 10-20%. These carbides may be dissolved into fcccarbides up to about 10-20% without disrupting the cubic structure. Allpercentages referred to herein are mole percents.

The semiconductor device base comprises a suitable substrate and a3C-SiC overlay epitaxially related to the substrate, said overlay beingunpolytyped, single crystal, uncracked, free from twins, and possessingintegrated circuit quality surface morphology. Methods of preparing theepitaxial overlay include reactive evaporation, chemical vapordeposition, and liquid phase epitaxy, with the chemical vapor depositionmethod taught in U.S. Pat. No. 4,923,716 being especially useful. Bothsubstrate and 3C-SiC overlay may be doped to achieve regions ofdifferent electronic properties. Doping can be accomplished by knownmethods such as ion implantation or in-situ doping. The epitaxialrelation to the substrate may be direct or indirect. As used herein, theterm "epitaxially related to the substrate" includes the use ofinterlayers which facilitate a close lattice match between the substrateand the overlayer; "directly epitaxially related to the substrate"connotes no interlayer.

Devices may be fabricated into the silicon carbide by using epitaxialgrowth and dry etch techniques. Use of the device wafer base and processof the present invention permits the development of SiC devicetechnology, which has significant advantages at high temperatures, incorrosive environments, in the presence of radiation, and for highpower, high frequency devices. The device wafer base will permitfabrication of devices using simple modifications of well-known siliconfabrication techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a semiconductor device wafer base having a silicon carbideoverlay layer epitaxially related to a cubic carbide substrate.

FIG. 2 shows a schematic representation of a float zone crystallizationapparatus used for substrate preparation.

FIG. 3 shows a chemical vapor deposition apparatus for depositing theSiC overlayer.

DETAILED DESCRIPTION OF THE INVENTION

3C-SiC inherently possesses many of the characteristics most desirablein a high temperature semiconductor material. It has a large bandgap,high breakdown voltage, excellent thermal conductivity, and may be dopedby the same species that are used to dope silicon, to achieve similarresults. It is chemically stable as the cubic polytype up to about 2000°C., as compared with silicon, which has a melting point of about 1420°C. 3C-SiC is highly resistant to diffusion of foreign atoms and ischemically inert to many reactants which can severely attack silicon andother common semiconductor materials. Ohmic contacts may be readily madeto 3C-SiC by techniques well known in the field of silicon semiconductortechnology, as by the method taught in U.S. Pat. No. 4,738,937, "Methodof Making Ohmic Contact Structure." FIG. 1 illustrates the type ofdeposited structure that may be formed with the process of theinvention. A semiconductor device wafer base 10 has a 3C-silicon carbideoverlay layer 12 epitaxially deposited upon a substrate 14. Thesubstrate 14 may be any of the single crystal alloys described in detailbelow. The illustrated wafer base 10 is not itself a semiconductordevice, but is typically further processed to form useful semiconductordevices thereon. The present invention is preferably employed to depositlayers of silicon carbide, but also may be used to form free standingsilicon carbide structures.

By way of illustration of possible approaches, a second overlay layer 16and a third overlay layer 18 may optionally be deposited overlying thesilicon carbide overlay layer 12. The second overlay layer 16 might be,for example, a layer of silicon carbide doped to be p-type or n-type.The third overlay layer 18 might be, for example, another layer of3C-silicon carbide or another doped layer. Such layered structures canbe developed by simply altering the composition of the gas flow to thedeposition apparatus.

Silicon carbide may exist in numerous polytypes, and interconversionbetween these polytypes can present problems for electronic devicefabrication. Crystallographic changes between polytypes during heatingor cooling result in variable bandgaps, transformational strains, andthe formation of defect structures, e.g. twins. Therefore polytypestability is required for use in electronic devices. 3C-SiC cubic hasonly one polytype and is more stable than the alpha polytypes. Thepresence of small amounts of alpha polytypes adversely affects 3C-SiCsemiconductor performance. Therefore, the techniques by which siliconcarbide devices are fabricated and used must ensure that polytypestability is achieved, most preferably the 3C-SiC cubic polytype.

For the present invention, the requirements for a suitable substrateare:

(1) The substrate must possess a cubic crystal structure, with latticeparameter within about 5% and preferably within 1% of that of 3C-SiC atambient temperatures.

(2) The substrate must undergo no significant phase, polytype orcrystallographic transformations from ambient temperature to above about1500° C. Thus the substrate must have a high melting or dissociationtemperature, above about 1500° C.

(3) The substrate must have high thermal conductivity, adequate toprovide a good thermal sink for extraction of heat from the 3C-SiC layerduring device operation.

(4) The substrate must be chemically inert to high temperatures, 1500°C. or higher.

(5) The substrate should have a coefficient of thermal expansion equalto or greater than that of 3C-SiC, thus allowing the overlay to befabricated at relatively high temperature and cooled with the overlay inmild lateral compression at ambient temperature. Compression is thoughtto prevent the formation of microcracks.

(6) The substrate constituents must be congruently melting, or seriousdifficulties will be encountered in efforts to fabricate it as a singlecrystal. If the material is not congruently melting, composition orcrystallographic fluctuations are likely to result uponrecrystallization.

(7) Finally, the substrate must be capable of fabrication insufficiently large single crystals such that a laterally extensive3C-SiC overlay layer is possible.

The use of titanium carbide went a long way toward addressing the firstsix criteria listed above. The last criterion has proven the mostdifficult to fulfill. We have now found that alloying the cubictransition metal carbides enables growth of large single crystals thatare very suitable for use as 3C-SiC substrates. The alloys may befabricated as large single crystals, in contrast to previously usedsubstrates such as pure TiC, which has proven very difficult to prepareas a single crystal without cracks, pinholes or dislocations. To date,it has been impossible to prepare pure TiC single crystals of 0.5 inchdiameter without defects, dislocations, cracks, or pinholes, and foreconomical device fabrication processes, diameters of about 2 inchesmust be achieved.

It is an object of the present invention to provide single crystalsubstrates that can be produced in large diameters without defects,dislocations, cracks, or pinholes. Solid solutions of transition metalcarbides have been long known. The transition metal carbides aremutually soluble in all concentration ranges. Transition metal carbidealloys melt higher at higher temperatures than does silicon carbide andhave a more negative free energy of formation. Therefore they areexpected to be more stable in every temperature range. Congruent meltingis expected in the composition range of metal:carbon of 0.8-1.0. Thermalconductivity may be closely matched. Lattice parameter mismatch may besmall. In the cubic carbides, the lattice parameter is a linear functionof mole percent composition. Therefore, the lattice parameter can be"engineered." The cubic crystal structure is retained, even uponalloying with hexagonal carbides, provided the hexagonal carbide ispresent in concentration below about 10-20%. The carbides which areuseful for forming alloys in this invention and properties which arerelevant to the device wafer base application are tabulated below(values were taken from "Refractory Materials," CRC Handbook ofChemistry and Physics, 63rd Edition, and Ullmann's Encyclopedia ofIndustrial Chemistry, Fifth Edition, VCH, Wernheim, Germany, 1986).

    ______________________________________                                        Suitable Metal Carbides                                                              crystal        lattice     melting                                     carbide                                                                              form           constant (Å)                                                                          point(°C.)                           ______________________________________                                        WC     hexagonal                  2630                                        VC     cubic          4.165       2830                                        MoC    cubic or hexagonal*                                                                          4.28        2695                                        TiC    cubic          4.33        3160                                        3C-SiC cubic          4.359       2100                                        TaC    cubic          4.455       3880                                        HfC    cubic          4.468       3890                                        NbC    cubic          4.461       3500                                        ZrC    cubic          4.698       3030                                        ______________________________________                                         *three hexagonal and one cubic polymorph exist                           

The benefits of alloying the transition metal carbides before preparingthe single crystal substrate are many. Disclocations do not travelthrough a mixed lattice as easily as through a pure crystal. Compressiveyield strengths of the alloys are higher, as taught by Hollox et al. inU.S. Pat. No. 3,661,599. Thus the alloys are better able to withstandthe high stresses inherent in the float zone recrystallization processwithout cracking. In some cases, alloying enables crystal growth atlower temperatures than with pure TiC, which makes processing easier.

Suitable alloy species for the present invention include alloys of twoor more of TiC, VC, NbC, TaC, ZrC, HfC, WC, and MoC. Alloys of VC andTiC can be prepared which have high compressive yield strength and closelattice match to 3C-SiC. The TiC melting point is 3160° C.; the VCmelting point is 2830° C. Therefore the alloy can be float zonerecrystallized at lower temperature than pure titanium carbide. Aniobium carbide:vanadium carbide alloy in a mixture of 2:1 NbC:VC moleratio would have exact lattice match to 3C-SiC. However, its meltingpoint would be higher. A quaternary alloy with TiC having compositionx(TiC)+(1-x)(2NbC:VC) has lower melting point. This alloy would retainan excellent lattice and thermal expansion match to 3C-SiC and have allthe advantages of ease of float zone recrystallization.

WC or MoC form hexagonal crystal structures in their pure states. Thesecarbides may be dissolved into cubic carbides up to about 10-20% withoutdisrupting the cubic structure. Examples of suitable alloys includeWC/TiC alloys, TaC/WC/TiC quaternary alloys, MoC/TiC/NbC (a compositionhaving 0.1:0.72:0.18 mole ratio would have an exact lattice match to3C-SiC).

Single crystal substrates and epitaxial overlays may be prepared byknown techniques. The single crystal crystal substrates may be preparedby float zone recrystallization. The epitaxial 3C-SiC overlay can begrown by known methods such as chemical vapor deposition or reactiveevaporation.

High pressure float zone crystal growth has historically been the methodof choice for producing single crystals of the transition metal carbidesbecause of their high melting temperatures. Float zone crystal growthinvolves passing a molten zone vertically through a polycrystallinepreform. If the molten zone moves at an appropriate rate, a singlecrystal solidifies behind the molten zone. FIG. 2 shows an apparatus 20for float zone recrystallization. The stainless steel shaft 21 isaffixed to a holder 22, usually constructed of molybdenum, with a boronnitride liner 23. The feed rod 24 is encircled by and passes through theinduction coil 25, which heats the feed rod to its melting point. Themolten zone of the feed rod solidifies as a single crystal 26 as itpasses out of the induction coil area.

For growth of metal carbide alloy single crystals, the preform isprepared from a metal carbide powder mixture of defined composition. Thecarbon content is adjusted to the range of atomic ratio of 1.2:1 to 1:1in order to be within the single phase region (which can be approachedfrom either the metal-rich or carbon-rich side). The average diameter ofthe powder grains is typically 1.0-5.0 microns. The preform is hotpressed. The hot pressing temperature is typically 1900°-2200° C.Alternatively, the preform may be isostatically pressed followed by asintering step. The melt is formed by induction heating, and the processis conducted under several hundred psi of helium because titaniumcarbide has a high vapor pressure at its melting point. The parametersthat are adjusted for optimal float zone growth are growth rate (rate atwhich the molten zone moves along the rod, gas cover, seed crystalrotation rate, rod rotation rate, and ingot diameter. The parameters fora typical metal carbide float zone recrystallization are as follows:

Growth Rate: 0.25 inch/hr.

Gas Cover: 600 psi He.

Seed Rotation: 3 rpm counterclockwise.

Rod Rotation: 4 rpm clockwise.

Ingot Diameter: 0.75 inch.

The epitaxial thin film of 3C-SiC may be deposited onto the preparedsubstrate by chemical vapor deposition as described in U.S. Pat. No.4,923,716 (Parsons & Brown, 1990). The silicon carbide layer 12 shown inFIG. 1 is deposited upon the substrate by chemical vapor deposition(CVD). A preferred apparatus 30 for accomplishing the CVD is illustratedin FIG. 3. In the apparatus 30, the substrate 14 is attached to agraphite susceptor 32. The susceptor and attached substrate are placedinto a chamber 34 within an RF (radio frequency) heater coil 36, wherebythe substrate 31 is heated to the deposition temperature as thesusceptor is heated. The chamber 34 is of a vertical double walledconstruction, and in operation cooling water is passed through the outerjacket 38. The reactive source gas is introduced through a port 39 atthe lower end of the chamber 34 and contacted with the substrate 14, sothat a silicon carbide overlay layer 12 is epitaxially deposited uponthe substrate 14 as a result of the pyrolysis of the source gas at thesurface of the heated substrate. As taught by U.S. Pat. No. 4,923,716,the preferred reactive gases are 1,2-disilylethane (also sometimes knownas 1,2-bis(silyl)ethane), disilylethene, disilylethyne, asilacycloalkane of the formula (SiH₂ CH₂)_(p), where p is 2, 3, 4, or 5,and a cyclic structure of the formula (CH₃ SiH)_(q), where q is 4 or 5.Disilylethane has a chemical formula of H₃ SiCH₂ CH₂ SiH₃. Disilylethynehas a chemical formula of H₃ SiC=CSiH₃. Of these preferred sourcematerials, 1,2-disilylethane is the most preferred. Alternatively, asilicon-containing gas such as silane and a carbon-containing gas may bereacted together over the substrate, but with separate sources of Si andC, the stoichiometry of the 3C-SiC film is more difficult to control.Halogen-containing source reagents can been used for CVD of 3C-SiC inthe practice of the invention, with methyltrichlorosilane beingespecially useful among these.

The epitaxial thin film of 3C-SiC may alternatively be deposited ontothe prepared substrate by reactive evaporation, for example as describedin U.S. Pat. No. 4,767,666 (Bunshah et al. 1988). Reactive deposition ata temperature of greater than about 1250° C., and preferably above about1325° C., may be used to epitaxially deposit the 3C-SiC overlay onto thesubstrate. In such a process, the substrate is placed into a vacuumchamber and pumped down, and then a flow of carbon-containing gas isestablished over the substrate. Silicon is evaporated from a sourcewithin the chamber, so that the evaporated silicon atoms pass throughthe carbon-containing gas and react therewith, and the resultingreaction products are deposited as an epitaxial layer on the substrate.The preferred carbon-containing gas is acetylene, although othercarbon-containing gases may be employed.

Liquid phase epitaxy is yet another technique which can be used in thepractice of this invention to deposit the epitaxial thin film of 3C-SiCas taught in U.S. Pat. No. 4,614,672, "Liquid Phase Epitaxy (LPE) ofSilicon Carbide." In this technique, the substrate is placed in a moltenbath of Si saturated with SiC at a temperature of between 1600° C. and1800° C. and 3C-SiC is deposited by reducing the temperature of thesubstrate to a temperature 20°-50° C. less than the molten Si.

It may be possible in the broad practice of the invention to use othermaterials with cubic crystal structure as the epitaxial overlay. Othermaterials with cubic crystal structure that might advantageously be usedas the epitaxial overlay include boron phosphide (lattice constant4.53Å) and zinc oxide (lattice constant 4.58Å).

EXAMPLES EXAMPLE 1. PREPARATION OF 5% VC/95% TIC ALLOY SINGLE CRYSTALSUBSTRATE

Vanadium carbide, vanadium, titanium carbide, and titanium powders witha particle size of 1 to 10 microns are blended together in the followingproportions:

VC: 481 gms.

V: 43 gms.

TiC: 8702 gms.

Ti: 773 gms:

The blended powders are placed in a graphite die to be hot pressed intoa rectangular plate with one dimension of the rectangle equal to thelength of the desired carbide preform, typically 6 inches. Sufficientpowder is added to the die to produce a plate whose width is that of thedesired substrate diameter, in this case 0.75 inches. The powder is hotpressed at approximately 2000° C. with a pressure of approximately 1700psi for approximately 30 to 60 minutes. The pressing is performed underargon gas. A plate having a uniform density of greater than or equal to95% of theoretical density is produced.

The hot-pressed plate is sliced into bars with a square cross sectionhaving dimensions of 0.75 inches×0.75 inches×6.0 inches. Using diamondtooling, the bars are centerless ground into cylindrical rods having adiameter of 0.5 inches and a length of 6.0 inches. These rods will beused as float zone preforms.

To produce a single crystal ingot, a rod is mounted vertically in afloat zone crystal growing apparatus. Each end of the rod is held by agrip which is capable of rotation and vertical translation. The gripsare composed of molybdenum lined with boron nitride. The molybdenum isattached to a water-cooled stainless steel shaft. A double pancakeinduction coil composed of two sets of two coplanar loops is positionedaround the bottom end of the preform rod, but at least one inch awayfrom the grip. The coil has an internal diameter of 0.75 inches, anoutside diameter of 1.75 inches, and a height of 0.600 inches. The coilis fabricated from 0.1875 inche diameter copper tubing. Water at apressure of 160 psi is flowed through the coil during the float zoneoperation to keep the coil cool.

The assembly is positioned in a chamber containing 600 psi of purehelium. A molten zone is created in the preform rod by the applicationof approximately 50 KW of radio frequency power with a frequency of 400KHz. The height of the molten zone is typically 0.5 inches. The diameterof the molten zone is typically 0.4 inches. Once the molten zone isestablished, the top grip is rotated clockwise and the bottom grip isrotated counterclockwise. Both grips are also moved downward at a speedof 0.25 inches per hour. This process is continued until an ingot of 2.5inches in length is created. The grips are then stopped, and power isreduced gradually over a period of several hours.

The resulting single crystal ingot is then oriented by X-ray diffractionto facilitate the slicing of wafers with a known crystallographicorientation. The <111> orientation is preferred for 3C-SiC epigrowth. AnID diamond wafering saw is typically used for slicing. Wafers having athickness of 0.5 to 1.0 mm are preferred.

The wafers are polished using a standard lapping wheel. Diamond pastesare used for rough polishing. The final polish is accomplished throughthe use of a mixture of 15.0 gms potassium ferricyanide, 2.0 gmspotassium hydroxide, 50 gms 0.05 micron alumina powder, and 450 ml ofdistilled water. This material has a lattice parameter of roughly 4.32 Åand has the face centered cubic crystal structure.

EXAMPLE 2. PREPARATION OF 50% VC/TIC ALLOY SUBSTRATE

The same process is used as in Example 1, with the exception that thecomposition of the powder blend was as follows:

VC: 4704 gms.

V: 423 gms.

TiC: 4476 gms.

Ti: 398 gms.

This material has a lattice parameter of 4.25 Å and has the facecentered cubic crystal structure.

EXAMPLE 3. PREPARATION OF NBC/VC/TIC ALLOY SUBSTRATE

The same process is used as in Example 1, with the exception that thecomposition of the powder blend was as follows:

VC: 1556 gms.

V: 140 gms.

TiC: 3702 gms.

Ti: 329 gms.

NbC: 3890 gms.

Nb: 383 gms.

This material has a lattice parameter of 4.34 Å and has the facecentered cubic crystal structure.

EXAMPLE 4. PPEPARATION OF TAC/VC ALLOY SUBSTRATE

The same process is used as in Example 1, with the exception that thecomposition of the powder blend was as follows:

TaC: 7456 gms.

Ta: 777 gms.

VC: 1622 gms.

V: 146 gms.

This material has a lattice parameter of 4.34 Å and has the facecentered cubic crystal structure.

EXAMPLE 5. DEPOSITION OF 3C-SIC ONTO 5% VC/TIC SUBSTRATE

The apparatus of FIG. 3 was used to deposit undoped 3C-silicon carbideupon the <001> surface of a substrate having composition 5% vanadiumcarbide: 95% titanium carbide. The temperature of the substrate wasvaried between 1200° C. and 1500° C., without a noticeable effect upondeposition. Hydrogen carrier gas was bubbled through liquid1,2-disilylethane maintained at 0° C. The total gas pressure was 1atmosphere, and the flow rate of the hydrogen was varied from 1 to 10sccm. Diluent hydrogen gas was also introduced along with the hydrogenbubbled through the bubbler to collect 1,2-disilylethane, for a totalgas flow of about 2900 sccm. 3C-Silicon carbide was deposited over theentire range of flow rate and temperature. The growth rate of thesilicon carbide was linearly related to the flow rate of hydrogenthrough the bubbler over the range studied. The 3C-silicon carbidegrowth rate was about 0.6 micrometers per hour, for each sccm ofhydrogen flow through the bubbler. The undoped 3C-silicon carbide wasmeasured to have an n-type carrier concentration of about 5×10¹⁶ percubic centimeter. This concentration is believed to be due to backgroundnitrogen donors.

EXAMPLE 6. DEPOSITION OF 3C-SIC ONTO 50% VC/TIC SUBSTRATE

The method of Example 5 was used to deposit undoped 3C-silicon carbideupon a (001) substrate having composition 5% vanadium carbide: 95%titanium carbide.

EXAMPLE 7. DEPOSITION OF P-DOPED 3C-SIC ONTO 50% VC/TIC SUBSTRATE

3C-silicon carbide with p-type doping was prepared using the sameapproach as described in Example 5, except that a dopant source gas wasmixed with the silicon carbide source gas and diluent gas. In thisexample, the hydrogen flow through the 1,2-disilylethane silicon carbidesource liquid was 10 sccm. A separate hydrogen flow of 1 sccm wasbubbled through liquid trimethylaluminum maintained at 20° C., so thattrimethylaluminum vapor was transferred to the deposition chamber alongwith the 1,2-disilylethane and the carrier gas. Aluminum was depositedin the 3C-silicon carbide as a dopant. The total gas flow was maintainedat about 2900 sccm.

The dopant level was linearly related to the gas flows. For example, adoubling of the flow rate of hydrogen through the dopant sourcematerial, while maintaining constant the flow rate through the siliconcarbide source, doubles the concentration of dopant in the 3C-siliconcarbide. Doubling of the flow rate of hydrogen through the siliconcarbide source bubbler, while maintaining the flow rate of hydrogenthrough the dopant source material bubbler, results in a halving of theconcentration of the dopant. The doped silicon carbide material wasmeasured to have a p-type carrier concentration of about 2×10¹⁹ percubic centimeter.

EXAMPLE 8. DEPOSITION OF N-DOPED 3C-SIC ONTO 50% VC/TIC SUBSTRATE

3C-silicon carbide with n-type doping was prepared using the sameapproach as described in Example 5, except that a dopant source gas wasmixed with the silicon carbide source gas and diluent gas. In thisexample, the hydrogen flow through the 1,2-disilylethane silicon carbidesource liquid was 5 sccm. A separate dopant flow of 10 sccm hydrogencontaining 46 ppm (parts per million) of NH₃ was mixed into the gasflow, so that nitrogen was transferred to the deposition chamber.Nitrogen was deposited in the 3C-silicon carbide as a dopant. The totalgas flow was maintained at about 2900 sccm. The linear dopingcharacteristics are found for this dopant, also. The n-type carrierconcentration of the doped 3C-silicon carbide was found to be about1×10¹⁸ per cubic centimeter.

The device wafer base and process of the present invention offer manyfunctionally significant advantages. Proper selection of substratematerial offers the capability of forming epitaxial overlayers of 3C-SiCof lateral size adequate for processing arrays of devices. The 3C-SiCmay be prepared substantially free of defects. The resulting devicewafer base is chemically and physically stable and incorporates the manyadvantages of 3C-SiC enumerated above. Various modifications arepossible within the spirit and scope of the present invention.

What is claimed is:
 1. A semiconductor wafer base comprising:a singlecrystal substrate having cubic crystal structure, said substratecomprising a material selected from the group consisting of alloys oftwo or more of TiC, VC, NbC, TaC, ZrC, HfC, WC, and MoC; and anunpolytyped, single crystal 3C-SiC overlay epitaxially related to saidsubstrate.
 2. The wafer base of claim 1, wherein the thickness of saidoverlay is about 0.05 to 100 micrometers.
 3. A semiconductor wafer basecomprising:a single crystal substrate having cubic crystal structure,said substrate comprising an alloy of TiC and VC with VC mole % rangingfrom 5-90%; and an unpolytyped, single crystal 3C-SiC overlayepitaxially related to said substrate.
 4. A semiconductor wafer basecomprising:a single crystal substrate having cubic crystal structure,said substrate being selected from the group consisting of alloys of twoor more of TiC, VC, NbC, TaC, ZrC, HfC, WC, and MoC; and an unpolytyped,single crystal overlay epitaxially related to said substrate, saidoverlay layer having a cubic crystal structure and comprising an alloyof SiC and one or more of TiC, HfC, ZrC, VC, TaC, MoC, WC, NbC, and AlN.5. A semiconductor wafer base comprising:a single crystal substratehaving cubic crystal structure, said substrate comprising a materialselected from the group consisting of alloys of two or more of TiC, VC,NbC, TaC, ZrC, HfC, WC, and MoC; and an unpolytyped, single crystal3C-SiC overlay epitaxially related to said substrate, wherein theoverlay is doped.
 6. A semiconductor wafer base comprising: a singlecrystal substrate having cubic crystal structure; and an unpolytyped,single crystal 3C-SiC overlay epitaxially related to said substrate,wherein the single crystal substrate comprises an alloy of two or moreof TiC, NbC, TaC and VC, having a lattice parameter of from 4.25 to 4.45Å.
 7. A semiconductor wafer base comprising:a single crystal substratehaving cubic crystal structure; and an unpolytyped, single crystal3C-SiC overlay epitaxially related to said substrate, wherein the singlecrystal substrate comprises an alloy of TiC and WC, with WC contentranging from 2 to 10 mole %.
 8. A semiconductor wafer base comprising:asubstantially defect-free single crystal substrate at least 0.5 inchesin diameter having cubic crystal structure, said substrate comprising amaterial selected from the group consisting of alloys of two or more ofTiC, VC, NbC, TaC, ZrC, HfC, WC, and MoC; and an unpolytyped, singlecrystal 3C-SiC overlay epitaxially related to said substrate.
 9. Thewafer base of claim 8, wherein the substrate has a diameter of at least2 inches.